The characteristics and spectrum of F9 mutations in Chinese sporadic haemophilia B pedigrees

Abstract

Introduction Sporadic haemophilia B (HB) without obvious familial history poses challenges for genetic diagnosis and counselling. Aim To identify the F9 variants in sporadic HB patients and probe the origin of these de novo mutations. Method A total of 294 unrelated HB pedigrees sought genetic diagnosis were analysed in this single‐centre study. The F9 gene was analysed by direct sequencing, and AccuCopy technique was adopted to screen for gene copy number variations. Six short tandem repeats approximal or within F9 gene were applied for linkage analysis. Mosaicism of sequence variant was determined by ddNTP Primer Extension method. Results Sporadic HB patients constituted 36% (61/294) of cases enrolled in current study. The sporadic and familial HB patients shared similar spectrum of F9 variants, with single nucleotide substitution as predominant form of disease‐causing mutation and no mutation prone hotspot sites, including CpG dinucleotide sequences, had been identified. Majority of the mothers of sporadic HB patients were F9 mutation carriers (70%, 43/61), and most of them (95%, 41/43) had the inherited bleeding trait traced back to maternal grandfathers. Although most de novo mutations occur in germ cells, 2 maternal grandfathers, who had somatic mosaic mutations of F9, were also revealed to be the source of genetic variations identified in patients. In our cohort, FIX inhibitor incidence was 1%, developed only in patients carrying null mutations. Conclusion The diversity of F9 genetic variants and possible mosaicism of de novo mutation demand extensive study and more cautious in genetic counselling of sporadic HB.

Defining the molecular pathology and consequent phenotypes in Egyptian HB patients

“…Mutational screening of F9 gene in eleven clinically diagnosed Egyptian hemophilia-B patients and carrier mothers revealed four point mutations including two missense and two nonsense mutations that were correlating with phenotypic severity within the studied patients. Up to November 2018, the F9 gene mutation database (EAHD Coagulation Factor Variant Databases) has recorded a total of 1094 mutations in 3713 HB patients [7], more than 3940 unique mutations have been reported so far in HGMD, 2020 [6]; point mutations accounts for 73.1% and mutations within the serine protease domain (SPD) account for about 56.1% among different populations [5,[8][9][10][11][12][13][14][15][16][17]. In agreement with that, all the detected mutations within our studied cohort were point mutations, three out of four detected mutations are within the protease domain, the greater part of codons (280-451) coded by the largest exon in the F9 gene (exon 8).…”

“…We identified one missense mutation (NM_000133.3: c.676C>G; NP_000124.1: p.Arg226Gly) located in exon 6 in two HB patients (patients 1 and 2) who clinically presented with moderate phenotype. However, severe and moderate forms of the disease resulting from this mutation have been reported earlier in different populations like China, Turkey, and France [18][19][20]; this might be attributed to different ethnic backgrounds [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21].…”

Turkey Experience in Molecular Analysis of Hemophilia B: F9 Gene Mutation Spectrum and Genotype-Phenotype Correlation

“…294 HB ailesinde yaptıkları çalışmada yanlış anlamlı varyant taşıyan 164 olgunun %38'inde ağır, %50'sinde orta ve %12'sinde hafif fenotip bildirmişlerdir. 20 Aynı çalışmada "null" varyant taşıyan 122 olgunun %44,3'ünde ağır fenotip gözlenmiştir. Bir başka çalışmada ise "null" varyant taşıyan 39 olgunun ağır, yanlış anlamlı varyant taşıyan 67 olgunun ise %49,3'ünün orta ve 47,8'inin de ağır HB fenotipi gösterdiği bildirilmiştir.…”

Target capture next‐generation sequencing in non‐inversion haemophilia: an alternative approach

“…Haemophilia A (HA) and haemophilia B (HB) are two forms of an X‐linked monogenic hereditary disorder caused by mutations in the coagulation factor VIII ( F8 ) and factor IX ( F9 ) genes respectively 1–3 . The prevalence of the disorders is 1 in 5000 (HA) and 1 in 30 000 (HB) male births 4 , 5 .…”

“…To date, 2582 and 1112 unique variants have been associated with the majority of HA (https://www.LOVD.nl/F8) and HB (https://www.LOVD.nl/F9) cases respectively. Determining the causative genetic variants in families affected by haemophilia is important for pregnancy, neonatal management and to inform about the risks of forming a neutralizing antibody and bleeding 2 , 8 , 9 …”